This invention relates generally to radar altimeters, and more specifically, to methods and systems that reduce interference caused by at least one of an antenna leakage signal and a signal reflected from other equipment mounted on an air vehicle.
A radar altimeter typically includes a transmitter for applying pulses of electromagnetic energy, at a radio frequency (RF), and at regular intervals to a transmit antenna which then radiates the energy, in the form of a transmit beam, towards the earth's surface. A transmit beam from a radar is sometimes said to “illuminate” an area (e.g., the ground) which reflects (returns) the transmit beam. The reflected beam, sometimes referred to as a ground return, is received at a receive antenna of the radar altimeter. A signal from the receive antenna is processed to determine an altitude of the air vehicle incorporating the radar altimeter.
At very low altitudes (generally defined as from 0 to about 50 feet), for example, during landing and take-off, altimeter performance may be impacted by leakage paths. Specifically, interfering signals may result from a leakage path between the transmit and receive antennas of the radar altimeter. In normal radar altimeter operations, as described above, a transmit antenna transmits a signal towards the ground which reflects the signal. The receive antenna receives the ground reflected signal for processing to determine air vehicle altitude. A leakage path exists when a portion of the transmitted signal is directly received by the receive antenna without having been reflected by the ground.
Other interfering signals are transmit signals reflected from air vehicle surfaces and structures (e.g., wheel well doors and helicopter skids) and transmit signals reflected from other equipment mounted on air vehicle surfaces (e.g., communication antennas, forward looking infrared systems, and cameras). The receive antenna receives these types of signals after antenna leakage but before the ground return pulse returns from the ground.
These unwanted signals may cause the altimeter to: (1) momentarily break track (i.e., the leakage signal periodically inhibits tracking the ground return signal such as during a hover phase cancellation mode), (2) lock on the leakage signal and always indicate a “zero” foot altitude regardless of the altitude of the air vehicle, or (3) oscillate between tracking the leakage signal, which indicates a “zero” foot altitude, and the ground return signal when the altitude is between 20 feet and 80 feet.
In helicopters and other air vehicles with the ability to hover, return signals reflected from various types of terrain such as grass, foliage, or even water can either add to each other or subtract from each other (i.e., phase addition or cancellation). During cancellation, the ground return signal is attenuated and is then more susceptible to interference from a stronger antenna leakage signal. Also, with new air vehicles containing more externally mounted equipment, it is becoming more difficult to find an acceptable antenna mounting arrangement that will maintain antenna leakage signal attenuation.
Solutions to interference signals may include different antenna placement, larger spacing between the antennas, different types of antennas, canting the antennas, or changing the location of other equipment that reflects signals back to the receive antenna. This can be very costly (e.g., changing the mounting or configuration of an air vehicle and potentially a number of the same model vehicles) as well as time consuming. Changing the mounting or configuration of an air vehicle can hold up shipment of new air vehicles or force users to ground air vehicles.